"I think it would be lovely to grow old together," Cameron, 28, said at the apartment they share in Scotland. "Definitely," Heaney, 22, added, as he kissed her affectionately on the forehead.

There is a name for this: genetic sexual attraction. In Scotland, as in the United States, it's illegal. And Heaney and Cameron have been charged with incest.

"There have been times before the court case came up when I've said, 'Look, we've got to stop this,'" Cameron said. "We never could."

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They were sentenced to nine months' probation and banned from any contact.

But the bond remained and they are now back together. The courts allow them to live together, but not sleep together.

"There is a consistent and necessary need to be close to each other physically," Cameron said. "To actually feel each other close."

If they give in to temptation, they'll be sent to jail because Cameron and Heaney share the same mother. But they grew up apart.

Cameron was put into foster care when he was just a toddler. Heaney was raised by their birth mother.

They met only once during childhood, a brief meeting arranged by social services. Then, two years ago, they reunited at a family reunion.

"I think the first time I saw Danielle I found her very attractive," Cameron said. "But I also thought, 'Hang on a second, this is your sister you're talking about.'"

At the time, Heaney was married, so she broached the subject with her husband, with whom she had a daughter: "I said I could grow to love Nick very easily. And funnily enough he said, 'Hopefully, just as a brother?' And I said, 'Of course!'"

Cameron and Heaney became close quickly, sharing secrets and having what they call "tickle fights."

"There were quite a lot of times we would just sort of stop and look into each other's eyes and I think both of us wanted to kiss each other at that point," Cameron said. "But of course we decided against that. And then one day I just decided to heck with it and I just kissed her."

That was a turning point for Heaney.

"I only realized I was falling in love with Nick after I kissed him," she said. When asked what made her kiss him, she replied with a smile, "Well, look at him, he's gorgeous!"

One psychotherapist explained how this could happen.

"When you see someone for the first time that you're related to there is this intense attraction," said Joe Soll, the author of "Adoption Healing." He says that the initial feeling is "emotional attraction that gets turned into sexuality sometimes."

Cameron and Heaney have gone public with their forbidden love in the hope that it will spark more research into genetic sexual attraction. "I think we need to have some understanding," Soll said. "To not blame them for getting carried away that way."

For Cameron and Heaney, their relationship became physical quickly.

"Our mother actually walked in on us," Cameron said, wincing at the memory. "And of course wasn't too happy. … I mean, I can understand her reaction."

He does not believe their mother, who is a devout Christian, will ever come to terms with their relationship.

In fact, it was their mother who called the police to report their incestuous relationship.

"At the time when all this was going on I was just thinking, well we're just reliving a childhood that we never actually had together," Cameron said.

But siblings share intimacy without sharing a bed. Why can't they?

"Any sort of sexual thing is a natural bonding process," Cameron said. "This need to be together could actually be a fear of being apart again because you haven't grown up together."

Another theory: The social taboo isn't an issue for them, psychotherapist Soll said. "If you don't grow up with your brother or sister," he said, "one doesn't have that in-built taboo that children do have who have grown up together."

The taboo exists for most of us, and incest is illegal in all but a handful of countries because children born of incest often suffer genetic abnormalities. The question of children is not one that Cameron and Heaney face. She was sterilized after suffering severe postnatal depression after the birth of the daughter she had with her husband. Her ex-husband has custody of their child.

Despite all the problems they face, Cameron and Heaney are determined to stay together. "I still feel that I've found my soul mate," Heaney said. "And I'm not letting him go for anything."